Dynamic reactorless high-frequency vapor lamp ballast

ABSTRACT

A control device and method of operation for a discharge lamp which would under normal operation display a runaway discharge characteristic. The control device constitutes a part of a lowimpedance power supply which is without any effective current limiting impedance, and is thus a high-efficiency device. A highfrequency discharge sustaining potential is applied across the discharge lamp at a repetition rate of at least about 500 times per second. A feedback control signal is generated based on a predetermined lamp operating condition, and this feedback signal is used to control the duty cycle of operation, with each period of potential application not exceeding about 1.8 milliseconds, and with the duty cycle always being less than unity. The repetition rate of potential application may be simultaneously varied with the duty cycle of a feedback signal so that a particular discharge lamp operates at an optimum repetition rate.

llnited States Patent [151 3,648,106

Engel et al. Mar. 7, 1972 [54] DYNAMIC REACTORLESS HIGH- 3,222,57212/1965 Powell, Jr. ..315/310 x FREQUENCY VAPOR LAMP BALLAST Joseph C.Engel; Robert T. Elms, both of Monroeville, Pa.

[72] Inventors:

[73] Assignee: Westinghouse Electric Corporation, Pittsburgh, Pa.

Feb. 24, 1970 Filed:

Appl. No.:

[56] References Cited UNITED STATES PATENTS 1/ 1969 Widmayer ..315/29112/ 1969 Mahler 10/1966 Ahmed et al...

7/1967 Powell, Jr.

7/1965 Powell, Jr. ..315/100 U Primary Examiner-Herman Karl SaalbachAssistant Exahriner-Saxfield Chatmon, Jr. Attorney--A. T. Stratton, W.D. Palmer and Walter G. Sutcliff [57] ABSTRACT A control device andmethod of operation for a discharge lamp which would under normaloperation display a runaway discharge characteristic. The control deviceconstitutes a part of a low-impedance power supply which is without anyeffective current limiting impedance, and is thus a high-efficiencydevice. A high-frequency discharge sustaining potential is appliedacross the discharge lamp at a repetition rate of at least about 500times per second. A feedback control signal is generated based on apredetennined lamp operating condition, and this feedback signal is usedto control the duty cycle of operation, with each period of potentialapplication not exceeding about 1.8 milliseconds, and with the dutycycle always being less than unity. The repetition rate of potentialapplication may be simultaneously varied with the duty cycle of afeedback signal so that a particular discharge lamp operates at anoptimum repetition rate.

15 Claims, 7 Drawing Figures BASE DRIVE N AMPLIFIER DRIVE AMPLlFlER BASESYNCHRONIZ-ING PULSE Patented March 7, 1972 3,648,106

3 Sheets-Sheet 1 POWER SWITCHING SUPPLY MEANS j FEEDBACK SIGNAL FIG IGENERATING MEANS LAMP L\(QI BASE P d/ DRIVE Q2 AMPLIFIER I RJN IEEIGLAMP d) 8%; FLIP-FLOP AMPLIFQZ L +E' SYNCHRONIZING PULSE B 04 b L L B g/QB D5 i/ I C2 RESET L) Tl D2 f Ell \03 BI I| CURRENT 1 CURRENT lGENERATING LAMP GENERATING LAMP R20 7-, MEANS MEANS g L I| I2 PowER 1 KI2| SUPPLY V Lrzr aAMPLlFlER 5 "g R2 21 VB A 12 FIG.4A.

fi lNvENToRs DIO Joseph C. Engel and -Rl2 Robert T. Elms. D9 BY IATTORNEY Patented March 7, 1972 v 3 Sheets-Sheet 2 TIME FIG.3.

T I I I 2 3 4 s F- FIXED FREQuE-cY-| FIXED DUTY CYCLE TIME BASE DRIVEAMPLIFIER BASE DRIVE AMPLIFIER M 1 s2 K RI? m4 OIS E -SZ R16 L 5E] g Q ELAMP Patented March 7, 1972 3 Sheets-Sheet 5 FIG. 5.

BACKGROUND OF THE INVENTION The standard commercial lighting dischargedevices, such as the fluorescent lamp and high pressure mercury vaporlamp, are characterized as negative resistance devices. During operationof these devices there exists a nonlinear relationship between thecurrent through the device and the voltage across the device. Thisnonlinear relationship necessitates controlling the lamp operatingparameters to stabilize the operation. In practice, reactive currentlimiting devices in series with the discharge device are generallyutilized to stabilize the operating device.

It has been recognized that the low-pressure mercury dischargefluorescent lamp device, as well as the high-pressure mercury dischargedevice, can be operated at high frequencies. The lamp types which can beutilized in the present invention have been characterized as exhibitinga runaway discharge under normal operation. This means that the lampcurrent will continue to increase to a destructive value without achange in lamp potential.

The reactive current limiting elements generally used in the prior artfor stabilizing discharge devices result in a significant portion of theenergy supplied being expended in controlling the discharge device. Theuse of such reactive control elements thus diminishes the overallefficiency of the lighting system.

SUMMARY OF THE INVENTION The present invention presents a control deviceand a method of operation for discharge lamps which does not includeprovision of any effective current-limiting impedance means in circuittherewith. A wide variety of discharge lamps which exhibit a runawaydischarge when normally operated from a power supply can be used inpracticing the invention.

The control device of the present invention constitutes part of alow-impedance power supply for operating the discharge lamps. Thecontrol device comprises, input terminals adapted to be connected to avoltage main, and output terminals adapted to be connected to thedischarge lamp. Low-impedance switching means are included which areoperable rapidly and repetitively, to directly electrically connect anddisconnect the input and output terminals at a repetition rate of atleast about 500 times per second. The control device has no effectivelamp current limiting means in circuit therewith. The switching meansare operated to establish a period of potential application which doesnot exceed about 1.8 milliseconds, as well as and an off period, thusdefining a duty cycle of operation. The duty cycle is the ratio of theperiod of connection of the input and output terminals to the totalperiod between successive connections of the input and output terminals,and this duty cycle is always less than unity. The control devicefurthermore comprises a feedback control means responsive to a lampoperating condition to generate a feedback output signal indicative ofthe lamp operating condition. Connection means are provided between theoutput of the feedback control means and the low-impedance switchingmeans to apply the feedback signal to the switching means operating sameto control or vary the duty cycle of operation to maintain thelamp-operating condition at about a predetermined desired level ofoperation. In the preferred embodiment of the invention thelamp-operating condition, which is monitored to generate the feedbackcontrol signal, is the average lamp wattage or power.

It has also been discovered that for each particular type of dischargelamp there is a preferred optimum repetition rate of potentialapplication which varies according to the impedance characteristic ofthe particular lamp. The repetition rate can be made variable and can becontrolled by an additional feedback signal so that the lamp will findits own preferred mode of operation. This repetition rate controleffected simultaneously with the stabilizing duty cycle control.

A generalized method of operation will be fully described in referenceto the exemplary apparatus set forth in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS The invention can be best understoodby reference to the exemplary embodiment illustrated in the following:

FIG. 1 is a block diagram outlining the general system;

FIG. 2 is a schematic of a specific embodiment utilized in practicingthe present invention;

FIG. 3 shows the various waveforms which are typically observed duringoperation of the system shown in FIG. 2;

FIG. 4 is a circuit which is a further embodiment of the presentinvention by which the repetition rate as well as the duty cycle can beautomatically controlled to effect stable lamp operation;

FIG. 4A is an embodiment of a circuit for producing signal 12 as shownin a block form in FIG. 4;

FIG. 5 shows the various waveforms which are produced during operationof the system shown in FIG. 4; and

FIG. 6 is an embodiment of a starting circuit for use with theembodiment of the invention in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT The general block diagram inFIG. 1 outlines the operation of the present invention. The power supplyand switching means constitutes a low-impedance potential source. Theinvention can be best understood by reference to the exemplaryembodiment shown in FIG. 2, and by a general explanation of itsoperation. The conventional discharge device I. comprises an at leastpartially light-transmissive envelope, and preferably has operativelyspaced electrodes disposed within the space enclosed by the envelope.The discharge-sustaining filling is disposed within the space enclosedby the envelope. The device L, is for example, a standard fluorescentlamp or a high-pressure mercury vapor device. The electrodes ofdischarge device L are connected across a bridge of highpowertransistors Q1, Q2, Q3, Q4, which are arranged to be diagonally drivento connect the potential of i E across the device. This transistorbridge comprises the low-impedance switching means. Terminal A of thetransistor bridge is connected via a variable resistor R1, which can becompletely bypassed or shunted after the startup procedure, to the highside of a potential source. Terminal B of the transistor bridge isconnected through the primary of the current transformer T1 to the lowside of the potential source. Tenninals A and B, thus comprise inputterminals adapted to be connected to a voltage main. The bases oftransistors Q1 and Q4 are connected to a common base drive circuit, andthe bases of Q2 and Q3 are also driven from a common drive circuit, withthe driving base currents being controlled as will be explained later.The node where the emitter of Q1 and the collector of Q3, and the nodewhere the emitter of O2 and the collector of 04 comprise the outputterminals of the control device.

In order to start up the higher impedance discharge devices of thepresent combinations, a certain level of ionization must be establishedin the discharge device. This is typically achieved by applying ahigh-voltage spike across the electrodes such as will be explained byreference to FIG. 6 later. The ionization can also be provided bydirecting a Tesla coil or other such high-frequency, high-voltage sourceproximate the device to initiate a discharge in the lamp. The presentlydescribed operation and control system can then take over operation ofthe device. Other starting arrangements can be readily utilized with thepresent system as will be apparent to one skilled in the art.

The present invention can be best described by reference to the voltagewaveforms shown in FIG. 3, which explain the operation of the detailedcircuit shown in FIG. 2. This description assumes that the dischargedevice is in operation and that R1 has been shorted out of the circuit.At time t1 at which time transistors 01 and Q4 are conducting or turnedon, the

potential, V, across the electrodes of the discharge device is +E. Acurrent i, is established in the discharge device L. It will be notedthat the current i, increases at a somewhat linear rate and the lampvoltage is constant during the time Q1 and Q4 are turned on. The currenti also flows through the current transformer T1. A logic signalproportional to the lamp current is induced in the secondary oftransformer TI. This logic signal flows from the dotted secondaryterminal of T1 through resistor R2, neglecting the small base current oftransistor Q5. The transformer T1 has a I to I turns ratio and the logicsignal produces a voltage across R2. Transistor O is connected as anemitter follower and thus the voltage across R2 and R3 are equal. DiodeD1 in series with R2 cancels the baseemitter drop of Q5. The emittercurrent and the collector current of Q5 then equals (R2/R3)Xl00) i,,/100) and flows from node C. A predetermined constant current value,provided by the basic transistorized current source formed by Q8, R5,the 5-volt Zener diode D and the potential source E, flows into node C.The current through the variable resistor R5 is 5/R5, and this constantcurrent acts as a reference value I The difference between the referencecurrent I and the logic signal current value, (R2/R3)((i /I00)), whichis the collector current of O5, is fed into the parallel configurationof Cl and R6. A voltage V A is established in this network which isproportional to the difference between the reference signal I and theaverage lamp current being controlled I, The filter time constant R6 Clis very long compared with the ontime period of the lamp voltage V andthus V is shown as a constant in FIG. 2.

The circuit thus far described provides an analog signal V which isbased on the average lamp current I,,. The remaining circuitry isbasically digital and converts this average lamp current error signal Vinto a control signal which is utilized to alter the on time and offtime of the transistors Q1 through Q4. Transistor Q6 operates as anemitter follower so that the current through R7 is V /R7. Neglecting thebase current of Q9, the collector current of Q6 will flow through C2.The reset circuit shown across C2 is simply a conventional solid stateswitch which resets the voltage on C2 to zero at the termination of eachon period of potential by application of a signal which is synchronouswith the application of V, The collector current of Q6 develops avoltage across C2 of V which increase linearly with time. V will beequal to V /C2R7 times the time T which is measured from the beginningof each half cycle. When voltage V reaches slightly more than 8 volts,the four-layer trigger diode D in the emitter circuit of Q9 fires, thusturning on Q9 and 07. These transistors 09 and Q7 remain on until V isreset to zero by a synchronized reset signal applied to the solid statereset circuit across C2. During the time that O7 is on the emittercurrent through R10 and R11 gives rise to an output voltage Y acrossR11. The voltage signal Y is combined with the output of a conventionalfree running flipflop, which has an output X and X, to provide a signalwhich is amplified, by a conventional pulse amplifier, here termed basedrive amplifier, and used to drive the transistors 01 through Q4. Thefree-running flip-flop of FIG. 2 can be a standard synchronized astablemultivibrator, the operation of which is described more fully inElectronics for Scientists" by H. V. Malmstadt and C. G. Enke, section9-8. The base drive amplifier component of FIG. 2 is more fullyexplained in, Transistor Circuit Design," Engr. Staff of TexasInstruments Inc., p. 245, McGraw-Hill, I963. A conventional integratedcircuit logic element is used to provide the control signal which isgenerated only when the voltage Y and X coincide, and when Y AND Xcoincide. The X AND Y signal which is transmitted only when there is avoltage Y coinciden t with X, is amplified and used to drive Q1 and 04while the X AND Y signal which is transmitted only when there is avoltage Y coincident with X is used to drive 02 and 03.

It can be readily recognized that as the average value of (i, increasesor decreases that V will be respectively decreased or increased invalue. The change in V will in turn affect the time for V to reach the 8volt breakdown level. Thus if (1) increases, V decreases, and the timein which V reaches 8 volts will be delayed. This will have the effect ofdelaying the establishing of voltage Y, so that the feedback controlsignal X AND Y is transmitted for a shorter period, i.e., Q1 and Q4 willbe driven to a conducting state later, the effect of which is to therebylower the average value of i,, back to the predetermined desired level.The signal X AND Y likewise drives Q2 and Q3 toward the same end.Another way of describing what is achieved by thus continuouslycontrolling the period of potential application and the interval periodbetween such applications is to describe the relationship in terms ofthe duty cycle. The duty cycle is the ratio of potential on time, i.e.,time during which potential is applied across the electrodes, torepetition time or the time from initiation of one potential pulse tothe next potential pulse. Thus, if the value of i is above apredetermined reference value when the device is operating with a givenduty cycle, the feedback signal generated will diminish the value of theduty cycle and this will have the effect of bringing i,, back to thepredetermined reference value. Again, it must be emphasized that thepresent method makes this adjustment of duty cycle and adjustment of theoperating condition value a continuous operation.

The invention can now be described in more specific terms. It has beendiscovered that the discharge device L can be a standard low-pressure,mercury discharge, fluorescent lamp, a high-pressure mercury vapor lamp,a metallic additive high pressure mercury vapor device, or ahigh-pressure sodiummercury discharge device. It should be understoodthat any discharge device which exhibits a substantially resistiveoperating characteristic at high pulse-repetition rates can be utilizedin practicing the present invention.

A standard 40-watt low-pressure mercury discharge fluorescent lamp isinserted as L in the circuit shown in FIG. 2. The desired averagelamp-operating current for this lamp is about 0.30 ap. The voltage E wasfound to be preferably I30 volts, and the nominal initial duty cycle ispreferably designed to be about 0.75. This lamp does not require the useof the variable load resistor R1 for starting purposes.

The nominal duty cycle can be set at a value which efficiently utilizesthe transistors by selecting the value of E. The duty cycle will becontinuously adjusted during operation. Initially the duty cycle can beset at about 0.75 by determining that the waveform outputs X and X ofthe flip-flop means each have a square wave duration of 0.2 millisecond.The synchronization signal output of the flip-flop is also generated at0.2 millisecond intervals. The adjustment of the duty cycle is made bythe control signals Y and X, and Y AND X, which are generated only whenV reaches the trigger potential of diode D in the emitter circuit of Q9,at which point Q9 and Q7 become conductive and voltage Y is generated.Since V Va/ C2R7 t, an adjustment of these parameters will effect thetime for establishing the voltage Y, and correspondingly the time forapplication of voltage across the lamp as is shown in FIG. 3, Duringoperation the duty cycle will be varied continuously as the lamp currentdeviates from the reference current, by varying the period of potentialapplication. The voltage waveforms X and X are preferably 0.2millisecond in duration each, and the system parameters should beadjusted to provide a 50- microsecond delay before V reaches thebreakdown potential of D to thus provide a 0.75 duty cycle.

The synchronization signal is used to reset V across C2 to zeroperiodically in synchronization with V,, going to zero as shown in FIG.3, and thus allowing continuous control during operation. The resetcircuit shown in FIG. 2 can be any of a number of solid state switcheswhich is triggerable by the synchronizing pulse to a closed position, tothus shunt C2, and make V go to zerol.

It has been discovered that whenL is a standard 40 watt fluorescentlamp, that the repetition rate of the potential application ispreferably from 1,000 to 40,000 times per second, at a reasonable dutycycle. For a slower repetition rate, three are consequent larger changesin current and average conductivity during the period of potentialapplication, which would result in nonstable operation.

As another example, a standard 400-watt high-pressure mercury vaporarc-discharge device as shown in FIG. 5 is lamp L in the diagram of FIG.2. The preferred value of E is 173 volts, and variable resistor R1 isset at approximately 75 ohms during a warmup period of several minutes,after which R1 is shunted out of the circuit. The duty cycle ispreferably designed at about 0.75. The reference current I can bereadily set at about 2.3 a. by adjusting the value of E and R5.

The repetition rate is preferably from about 2 to 5 thousand times persecond when the lamp being operated is a conventional 400-watthigh-pressure mercury vapor discharge. At higher repetition ratefrequencies a standing acoustical wave developes which impedes operationof this particular device.

The discharge device can also be a standard 400-watt metal halideadditive high-pressure mercury vapor lamp as per US. Pat. No. 3,234,421issued Feb. 8, 1966. In this case it has been found desirable to set Eat 154 volts. R1 is 60 ohms and is shorted after a warmup period ofseveral minutes. The duty cycle is preferably designed to be about 0.75.The reference current can be readily set at about 2.6 by adjusting thevalue of E and R5. In general, for metal halide additive devices, therepetition rate is preferably from about 2 to thousand times per second.

The discharge device can also be a standard 400-watt sodium-mercuryhigh-pressure device such as described. The

preferred value of E is 150 volts and R1 is set at about 55 ohms duringa warmup period of several minutes, after which RI is shunted out of thecircuit. The duty cycle is preferably designed to be about 0.75. Thereference current I can be readily set at about 2.67 a. by adjusting thevalue of E and R5. For such a discharge device the repetition rate ofpotential application is preferably from about 8 to. 40 thousand persecond.

While the lamp operating current was the parameter which was monitoredto effect the control of the present combination, other lamp-operatingconditions such as the light output as monitored by a photoresponsivemeans, or lamp power as monitored by a device such as described incopending application, Ser. No. 807,658, filed Mar. l7, I969, owned bythe assignee of the present invention.

In the preferred embodiment of the invention as described by the workingexample, the duty cycle was controlled by varying the turn on time ofpotential application, or stated another way, the pulse width of theenergization waveform was modulated. The duty cycle can also becontrolled by varying the repetition rate of the free running flip-flopsignals, or by varying both the pulse width and the repetition rate.

While the waveform potential applied across the discharge device in thecircuit shown in FIG. 2, is a reversing potential square wave it must bestressed that there need be no reversal of the potential, nor is itessential that the waveform signal be a square wave. The use of a squarewave pulse does keep the lamp current fairly constant and is thus thepreferred method of operation. The applied waveform could similarly be arepetitive pulse of the same polarity.

In practicing the present invention the control device is generallyprovided with means for providing a fixed repetition rate which ispredetermined for a given type of discharge device. In general, thehigher operating current and lower operating voltage device has a veryrapid response time, or stated another way the dynamic impedance variesin a shorter period of time, and the preferred repetition rate ofpotential application should have a higher frequency to maintain astable operating condition.

The dynamic impedance can be determined for various discharge devicesand the preferred repetition rate provided by fixing the frequency ofthe synchronizing pulse of the circuit of FIG. 2, which is derived fromthe flip-flop device.

It has been determined that a repetition frequency of at least about 500times per second for the application of the operating potential isrequired for lamps operated in accordance with the present invention.The actual control function, which compensates for variations in lampoperating characteristics, is carried out by continuously varying theduty cycle, and thus the duty cycle should always be less than unity.The total period of potential application in any one duty cycle must notbe so long as to permit a runaway discharge to occur, and it has beenfound that the period for application of operating potential should notexceed about 1.8 milliseconds.

It is sometimes desirable to be able to automatically operate a varietyof discharge devices, from a single fixture and control device. Thevarious discharge devices exhibit very different dynamic impedancecharacteristics and operate at an optimum at widely varying operatingvoltages and repetition frequencies.

The conductivity of a low-dynamic impedance device changes very rapidlyduring the period of potential application, and this very fast responsetime threatens to create an unstable runaway discharge, so thehigh-repetition rate which approximates the response time, minimizesdrastic conductivity changes and maintains a stable operating conditionfor the device.

The term dynamic impedance as used in this application means theinstantaneously determinable self-impedance of the discharge device. Theresponse time for changes of discharge impedance is the time required toreflect a discernable change in the dynamic impedance of the device. Thecontrol device of the present invention tailors the electrical inputapplied to the lamp to allow the discharge device to control itself.Changes in the dynamic impedance of the device are sensed by sensingsome lamp-operating condition indicative of same and the electricalinput is adjusted to maintain stable operation and control. A morespecific embodiment allows the control device to be used on any numberof different discharge devices which are observed to operate at anoptimum mode, at different repetition rates depending upon the specificimpedance characteristics of the given discharge.

In the embodiment of the invention shown diagrammatically in FIG. 4, thecontrol device not only includes means for continuously varying the dutycycle of potential applied to the discharge device, but also means forcontinuously varying the frequency of application of potential orrepetition rate of potential applied to the discharge device. Thislatter provision allows the control device to be used with dischargedevices of varying or distinctly different dynamic impedances or wherethe dynamic impedance of the discharge device varies widely over itslife. It has been discovered that there is an optimum range of frequencyapplication for discharge devices of specific dynamic impedancecharacteristic. The control device can have a variable repetition ratewhich is detennined by a feedback control signal which is generated bysensing a lamp-operating condition and comparing this to a predeterminedsignal value which is indicative of a predetermined desired operatingcondition, and thereby generating the feedback control signal.

In FIG. 4, I1 is a current signal proportional to the average powererror, which is bad for example by utilizing a transistorized wattmetersuch as described and explained in copending application, Ser. No.807,659, filed Mar. 17, 1969 by the present applicants, and owned by thepresent assignee.

The averaged signal produced by the transistorized watt meter can bereadily compared to a constant reference signal proportional to apredetermined desired average wattage, to generate II. This constantreference signal can be produced by a constant current generator asdescribed in the aforementioned copending application, with the constantcurrent node proportional to a desired average wattage for the lamp. Anexemplary circuit for the means for generating [2, as a function of thelamp voltage is shown in FIG. 4A. The resistors R20 and C7 produce anegative voltage proportional to average lamp voltage. The signal I2 isarrived at by summing the currents at node K.

In the circuit of FIG. 4, a current I1, is generated which isproportional to the average power error. The average lamp power ismonitored as was the average lamp current in the embodiment shown inFIG. 2. A signal is generated which is a function of the average lamppower, and this signal is compared to a reference signal whichcorresponds to the desired average power rating. An average power errorsignal is then applied to a controlled current source, and I1 is theresulting current which is proportional to the average power error.

The lamp voltage V is continuously monitored during operation, and thecircuit includes means for generating a current 12 which is a functionofthe lamp voltage.

The transistors Q and Q11 are arranged as differential amplifiercomponents. A pulsed output voltage signal is generated which has a dutycycle and repetition rate which is determined by the values of II and [2respectively.

The magnitude of 12 determines the rate of increase of V and when Vreaches 1; V the unijunction transistor Q12 fires and V goes to zero,and is thereafter recharged repetitively.

The pulsed output signal has a frequency of f=I /2"r; C3 V and a dutycycle =1 R1Il/"qV,,. Thus, by variations of 11, the duty cycle can becontrolled, and by variation of I2 the frequency can be controlled. Thewaveform diagrams of FIG. 5 demonstrate how the control is effected. Thepotential V is a sawtooth with the slope of the ramp portion being afunction of the magnitude of 1 The magnitude of signal 1 determines atwhat value of V an output pulse is initiated. Referring to FIG. 5,during the time interval from 1 to 2, I1 is constant at a value lowerthan the preferred level, which means there is no change in power errorfunction, so that the point at which the output signal pulse startsremains the same. Thus, 12 is constant which means the point when theunijunction Q12 fires and V goes to zero is the same, thus one hascontrolled turnon time and the frequency for the output signal. Thisnecessarily means that the off time and the duty cycle are determined.Now at time 2, a step increase in 11 takes place which corresponds to anincrease in power error signal which means that the turn-on time for theoutput signal will occur at a higher value of V Since I2 is stillconstant and the frequency is thus constant, the duty cycle isdecreased. At time 3, l2 increases in a stepped function manner and thusthe slope of the waveform V increases and there will be a change in therepetition frequency, but since 11 remains the same the duty cycleremains the same. At time 4, I1 increases above the preferred referencevalue and thus the turn on time will occur at a still higher value ofV,,, I2 decreases and thus the slope of waveform V,, is not as sharp andthe repetition frequency is reduced.

The output signal from the transistorized differential amplifier systemis used to drive the power amplifier, which comprises a power supply andtransistor bridge such as shown in FIG. 2, to power the dischargedevice.

In yet another embodiment of the invention as shown in FIG. 6, astarting circuit is shown which is used to deliver high voltage startingpulses to particular discharge devices exhibiting a high initialimpedance such as sodium-mercury amalgam discharge device, to insureinitial warmup to an operational impedance level. The circuit shown willallow the high voltage pulse to be applied to the discharge device, yetwill isolate the voltage from damaging the transistorized inverter. Thestarting circuit is only in operation when the temperature of thedischarge device is below a predetermined warmup level, above thiswarmup temperature no high-voltage pulses are generated and the startingcircuit plays no part in the operation of the control device.

The starting circuit shown in FIG. 6 comprises, the high voltage pulseforming means formed by R18, C4, D16, T2, C5, the lamp warmup currentpath formed by S2, D14, and Q13, the gate drive for Q13 through S2 andR17, and the gate drive through S2 and R19 for transistor Q17 which isused to control the base of transistor Q14, to keep Q14 turned offduring warmup. Switches S1 and S2 are temperature-sensitive vacuumbimetal switches. The high voltage switch S1 is normally open at ambienttemperature and serves to isolate transistors Q14 and Q16 of the controldevice transistorized inverter from the high-voltage starting pulses.The switch S1 is spaced proximate the operating discharge device so thatS1 closes when the discharge devices reaches a predetermined operatingtemperature after the warmup period. The switch S1 is between the outputportion of the pulseforming means and the low-impedance switching meansto isolate the low-impedance switching means from the side of thedischarge lamp to which the high-voltage pulses are applied. The switchS1 is responsive to a lamp-operating condition such as temperature, toclose and effectively remove operating potential across thepulse-forming network because of the low-impedance path through D15,closed S1, and the discharge lamp. The switch S2 is also spacedproximate the discharge device'and is normally closed at ambienttemperature. In one path S2 serially connects the operating potentialvia S2 and R17 to the base of transistor Q13, to drive Q13 on, and sincethe emitter of 013 is connected to the operating potential source thisprovides a current path for the wannup current via S2, D14, R16, thedischarge device, and Q13. This warmup current will flow when the highvoltage starting pulse is applied to the discharge device. When thedischarge device reaches a predetermined operating temperature, S2senses this and opens a predetermined time after S1 closes, thuseliminating the wannup current path, and removing the control of Q14.The primary transistorized control circuit is now able to perform asdescribed before in reference to FIG. 2. The base drive signals for thetransistorized bridge of FIG. 6 is derived as explained in describingFIG. 2. The circuit points M and N of FIG. 6 correspond to points A andB of FIG. 2.

We claim as our invention:

1. A control device which constitutes a part of a low-impedance powersupply for operating a discharge lamp without effective current-limitingimpedance means in circuit therewith, which lamp when normally operatedfrom a lowimpedance power supply will exhibit a runaway dischargecharacteristic, said control device comprising:

a. input terminals adapted to be connected to a voltage main, and outputterminals adapted to be connected to said discharge lamp,

b. low impedance switching means operable to rapidly and repetitivelydirect electrically connect and then disconnect said input terminals andsaid output terminals at a repetition rate of at least about 500 timesper second, said input and output terminals when directly electricallyconnected by said switching means having no effective lampcurrent-limiting impedance in circuit therewith, said switching meansoperable to maintain connection between said input terminals and saidoutput terminals for a time period not exceeding about 1.8 milliseconds,said switching means having a duty cycle of operation which constitutesthe time duration of any individual period of connection of said inputand output terminals divided by the time duration between successiveconnections of said input and output terminals, and said duty cyclealways being less then unity,

c. feedback control means responsive to a lampoperating condition togenerate at least one feedback output signal indicative of suchlamp-operating condition; and

d. connection means between the output of said feedback means and saidswitching means to apply said feedback signal to said switching means tooperate same to vary said duty cycle to maintain said lamp-operatingcondition at about a predetermined desired level of operation.

2. The control device as specified in claim 1, wherein saidlamp-operating condition is the average lamp wattage.

3. The control device as specified in claim 1, wherein said feedbackcontrol means compares a signal which is indicative of saidlamp-operating condition to a reference signal which is indicative of apredetermined desired level of lamp operating condition to generate saidfeedback output signal.

4. The control device as specified in claim 1, wherein said repetitionrate is fixed at a predetermined value, and said feedback signalcontrols said switching means to vary said duty cycle to maintain saidlamp-operating condition at about a predetermined desired level ofoperation.

5. The control device as specified in claim 1, wherein said dischargelamp is a low-pressure mercury vapor discharge lamp, and wherein saidrepetition rate is preferably from |,000 to 40,000 times per second.

6. The control device as specified in claim 1, wherein said dischargelamp is a high-pressure mercury vapor discharge lamp, and wherein saidrepetition rate is preferably from 2,000 to 5,000 times per second.

7. The control device as specified in claim 1, wherein said dischargelamp is a metallic halide additive high-pressure mercury vapor dischargelamp, and wherein said repetition rate is preferably from about 2,000 to20,000 times per second.

8. The control device as specified in claim 1, wherein said dischargelamp is a sodium-mercury amalgam high-pressure discharge lamp, andwherein said repetition rate is preferably from about 8,000 to 40,000times per second.

9. The combination as specified in claim 1, wherein said means forgenerating a signal which is a function of the average power input tosaid lamp comprises means for sensing a signal which is a function ofthe average current through said discharge lamp, and means for comparingthis sensed signal which is a function of the average current throughthe lamp to a fixed reference value which is representative of thedesired lamp current, and generating a signal which is representative ofthe difference between actual average current and desired averagecurrent.

10. The control device as specified in claim 1, wherein saidlow-impedance means comprise a plurality of transistors forming atransistorized inverter coupling said discharge lamp to a source ofoperating potential, with the feedback control output signal applied asthe base drive to said transistors.

11. The device as specified in claim 1, wherein starting means arecoupled to said low-impedance switching means, said starting meanscomprising;

a. pulse forming means connected across said input terminals forgenerating high-voltage starting pulses across said output terminalswhen-said pulse forming means is connected to a source of operatingpotential, and the resulting high voltage pulses being applied acrosssaid discharge lamp for lamp start-up;

b. a normally open high-voltage switching means between the outputportion of said pulse-forming means and said low-impedance switchingmeans to isolate said low-impedance switching means from saidhigh-voltage pulses, said high-voltage switching means responsive to apredetermined low-impedance lamp operating condition to close andeffectively remove operating potential from across said pulse-formingmeans;

c. lamp warmup network means comprising a normally closed switchingmeans and a current-limiting impedance serially connected with saidlamp, and said normally closed switching means responsive to apredetermined lamp-operating condition to open after said normally openhigh-voltage switching means closes, and said lowimpedance meansresponsive to opening of said normally closed switching means toinitiate operation of said lowimpedance switching means, whereby whensaid starting means has operating potential applied thereto, saidhighvoltage pulses are applied across said lamp and a warmup currentflows through said lamp, and after said lamp is warmed up, said normallyclosed switching means opens to isolate said warmup network from saidoperating potential.

12. The device asspecified in claim 1, wherein said signalrepresentative of average power input is proportional to the averagecurrent for said discharge lamp, and the feedback control signal isapplied to said switching means to vary the connection period of saidinput and output terminals, the disconnection period, and thus theresulting duty cycle.

13. The control device as specified in claim 12, wherein said feedbackcontrol signal varies both said repetition rate and said connectionperiod of said in ut and output terminals.

14. The combination as specr red in clarm 13, wherein sard feedbackcontrol means includes means for sensing the potential across thedischarge device and for generating a feedback signal which isindicative of said potential across the discharge device.

15. The method of operating a discharge lamp from a variablepower-output low-impedance potential source with an averagepredetermined power input, which lamp when normally operated from alow-impedance potential source will exhibit a runaway dischargecharacteristic, which method comprises:

a. energizing said lamp from a variable power-output lowimpedancepotential source, with no effective lamp ballasting impedance in circuittherewith, with pulsed-type excitation having a repetition rate of atleast about 500 times per second, the duration of any individualexcitation pulse not exceeding about 1.8 milliseconds, and the dutycycle which constitutes the time period for which any individual pulseis applied divided by the time period defined by the initiation of suchexcitation pulse to the initiation of the next succeeding pulse alwaysbeing less than unity, and said variable power-output low-impedancepotential source responsive to a control signal to vary said duty cycleto vary the power output thereof;

b. monitoring a predetermined operating characteristic of the operatinglamp to generate a control signal which is indicative of whether theoperating lamp has an average wattage input thereto which is less thandesired, or more than desired, or is at about that wattage input theretowhich is desired;

c. applying the control signal to the low-impedance potential source tovary the duty cycle to vary the average power input to the operatinglamp as required to maintain such average power input at about itspredetermined desired value.

1. A control device which constitutes a part of a low-impedance powersupply for operating a discharge lamp without effective current-limitingimpedance means in circuit therewith, which lamp when normally operatedfrom a low-impedance power supply will exhibit a runaway dischargecharacteristic, said control device comprising: a. input terminalsadapted to be connected to a voltage main, and output terminals adaptedto be connected to said discharge lamp, b. low impedance switching meansoperable to rapidly and repetitively direct electrically connect andthen disconnect said input terminals and said output terminals at arepetition rate of at least about 500 times per second, said input andoutput terminals when directly electrically connected by said switchingmeans having no effective lamp current-limiting impedance in circuittherewith, said switching means operable to maintain connection betweensaid input terminals and said output terminals for a time period notexceeding about 1.8 milliseconds, said switching means having a dutycycle of operation which constitutes the time duration of any individualperiod of connection of said input and output terminals divided by thetime duration between successive connections of said input and outputterminals, and said duty cycle always being less then unity, c. feedbackcontrol means responsive to a lamp-operating condition to generate atleast one feedback output signal indicative of such lamp-operatingcondition; and d. connection means between the output of said feedbackmeans and said switching means to apply said feedback signal to saidswitching means to operatE same to vary said duty cycle to maintain saidlamp-operating condition at about a predetermined desired level ofoperation.
 2. The control device as specified in claim 1, wherein saidlamp-operating condition is the average lamp wattage.
 3. The controldevice as specified in claim 1, wherein said feedback control meanscompares a signal which is indicative of said lamp-operating conditionto a reference signal which is indicative of a predetermined desiredlevel of lamp operating condition to generate said feedback outputsignal.
 4. The control device as specified in claim 1, wherein saidrepetition rate is fixed at a predetermined value, and said feedbacksignal controls said switching means to vary said duty cycle to maintainsaid lamp-operating condition at about a predetermined desired level ofoperation.
 5. The control device as specified in claim 1, wherein saiddischarge lamp is a low-pressure mercury vapor discharge lamp, andwherein said repetition rate is preferably from 1,000 to 40, 000 timesper second.
 6. The control device as specified in claim 1, wherein saiddischarge lamp is a high-pressure mercury vapor discharge lamp, andwherein said repetition rate is preferably from 2,000 to 5, 000 timesper second.
 7. The control device as specified in claim 1, wherein saiddischarge lamp is a metallic halide additive high-pressure mercury vapordischarge lamp, and wherein said repetition rate is preferably fromabout 2,000 to 20,000 times per second.
 8. The control device asspecified in claim 1, wherein said discharge lamp is a sodium-mercuryamalgam high-pressure discharge lamp, and wherein said repetition rateis preferably from about 8,000 to 40,000 times per second.
 9. Thecombination as specified in claim 1, wherein said means for generating asignal which is a function of the average power input to said lampcomprises means for sensing a signal which is a function of the averagecurrent through said discharge lamp, and means for comparing this sensedsignal which is a function of the average current through the lamp to afixed reference value which is representative of the desired lampcurrent, and generating a signal which is representative of thedifference between actual average current and desired average current.10. The control device as specified in claim 1, wherein saidlow-impedance means comprise a plurality of transistors forming atransistorized inverter coupling said discharge lamp to a source ofoperating potential, with the feedback control output signal applied asthe base drive to said transistors.
 11. The device as specified in claim1, wherein starting means are coupled to said low-impedance switchingmeans, said starting means comprising; a. pulse forming means connectedacross said input terminals for generating high-voltage starting pulsesacross said output terminals when said pulse forming means is connectedto a source of operating potential, and the resulting high voltagepulses being applied across said discharge lamp for lamp start-up; b. anormally open high-voltage switching means between the output portion ofsaid pulse-forming means and said low-impedance switching means toisolate said low-impedance switching means from said high-voltagepulses, said high-voltage switching means responsive to a predeterminedlow-impedance lamp operating condition to close and effectively removeoperating potential from across said pulse-forming means; c. lamp warmupnetwork means comprising a normally closed switching means and acurrent-limiting impedance serially connected with said lamp, and saidnormally closed switching means responsive to a predeterminedlamp-operating condition to open after said normally open high-voltageswitching means closes, and said low-impedance means responsive toopening of said normally closed switching means to initiate operation ofsaid low-impedance switching means, whereby when said starting means hasoperating potenTial applied thereto, said high-voltage pulses areapplied across said lamp and a warmup current flows through said lamp,and after said lamp is warmed up, said normally closed switching meansopens to isolate said warmup network from said operating potential. 12.The device as specified in claim 1, wherein said signal representativeof average power input is proportional to the average current for saiddischarge lamp, and the feedback control signal is applied to saidswitching means to vary the connection period of said input and outputterminals, the disconnection period, and thus the resulting duty cycle.13. The control device as specified in claim 12, wherein said feedbackcontrol signal varies both said repetition rate and said connectionperiod of said input and output terminals.
 14. The combination asspecified in claim 13, wherein said feedback control means includesmeans for sensing the potential across the discharge device and forgenerating a feedback signal which is indicative of said potentialacross the discharge device.
 15. The method of operating a dischargelamp from a variable power-output low-impedance potential source with anaverage predetermined power input, which lamp when normally operatedfrom a low-impedance potential source will exhibit a runaway dischargecharacteristic, which method comprises: a. energizing said lamp from avariable power-output low-impedance potential source, with no effectivelamp ballasting impedance in circuit therewith, with pulsed-typeexcitation having a repetition rate of at least about 500 times persecond, the duration of any individual excitation pulse not exceedingabout 1.8 milliseconds, and the duty cycle which constitutes the timeperiod for which any individual pulse is applied divided by the timeperiod defined by the initiation of such excitation pulse to theinitiation of the next succeeding pulse always being less than unity,and said variable power-output low-impedance potential source responsiveto a control signal to vary said duty cycle to vary the power outputthereof; b. monitoring a predetermined operating characteristic of theoperating lamp to generate a control signal which is indicative ofwhether the operating lamp has an average wattage input thereto which isless than desired, or more than desired, or is at about that wattageinput thereto which is desired; c. applying the control signal to thelow-impedance potential source to vary the duty cycle to vary theaverage power input to the operating lamp as required to maintain suchaverage power input at about its predetermined desired value.